This invention relates to laminates of reinforced phenolic resins coated with a final surface layer to remove surface irregularities.
Laminates comprising several successive layers of phenolic resins impregnated reinforcing agents are known in the art (see for example "Encyclopedia of Polymer Science and Technology", Interscience publishers 1970, Volume 12, page 1, "Reinforced plastics" and volume 8, page 121 "Laminates", and Modern Plastics Encyclopedia, 1975-76, volume 52, pages 153-158).
The phenolic resin generally used is a resol obtained by condensation of an aldehyde and a phenol in a basic medium, usually by condensation of formaldehyde and phenol in the presence of caustic soda. The curing or hardening of the resultant phenolic resin, i.e. cross linking in a three-dimensional system, is achieved by using an acid catalyst, for instance, hydrochloric acid, sulfuric acid, phosphoric acid, oxalic acid, benzenesulfonic acid, paratoluenesulfonic acid, phenol-sulfonic acid.
The reinforcing agent comprises fiber forms such as cellulosic fibers, glass fibers, non-woven materials made of large molecular weight polyesters or of polyvinyl chloride, or glass mats or woven material for example of aromatic polyamides, glass or asbestos.
The manufacture of reinforced laminates is achieved by any of several methods. According to one process, the reinforcing agent used in the form of a woven or non-woven material is impregnated with a resin; the impregnation is completed by using a roll mill or a press; then several impregnated layers of reinforcing agent are brought together. Laminates can be also obtained by winding of a filament impregnated with a resin. According to still another process, fibers are cast on a layer of resin previously poured on the surface of a mold, the pouring of the resin and casting of fibers being alternatively performed until the desired thickness is obtained.
Though well known, and a work horse of the industry since the 1920's, laminates based on phenolic resins still have significant drawbacks. In particular, they contain surface irregularities such as little holes, pocks, and fibers at and near the surface thereof projecting out over the surface. To obtain a smooth surface, it has been proposed to coat the laminate articles with a coating such as paint manufactured with a polyurethane binder, but the surface of the obtained article is not always smooth. A smooth surface can only be obtained by applying a primer coating or filler, e.g., a polyurethane resin, which plugs the pores, polishing the plugged surface and then painting the polished surface.
It has been also proposed to manufacture these laminates by pouring a resin layer on the surface of a mold, the resin layer being usually named "gel-coat". With a gel-coat comprising a thin layer of filled or unfilled phenolic resin, the resultant surface still exhibits irregularities such as small pits, and the imprint of the fibers also appears on the surface thereof. Whereas surface irregularities could theoretically be removed by using a thicker layer of phenolic resin, small cracks or crazing appears on the surface.
The same drawbacks are observed when using a gel-coat by an unsaturated polyester resin. Moreover, by using phenolic resins or unsaturated polyester resins, the resultant hardness of the surface becomes insufficient.